Switching power supply is a power conversion circuit that uses switching operations to convert an input voltage waveform into a specific output voltage waveform. A boost converter is an example of switching power supply that can provide power factor correction and total harmonic distortion reduction to the input voltage and convert the input voltage into a stable and regulated output voltage.
FIG. 1 is a circuit diagram of a well-known power factor correction (PFC) boost converter according to the prior art. In FIG. 1, a bridge rectifier BR is connected to an input AC voltage Vin for converting the input AC voltage Vin into a full-wave rectified DC voltage. A boost inductor L11 is connected to an output terminal of the bridge rectifier BR, and configured to store energy therein by receiving a current from the bridge rectifier BR and release the stored energy to an output capacitor C11 through a rectifying diode D11 according to the on/off operations of a switch Q11. Therefore, an output DC voltage is generated across the output capacitor C11. The output DC voltage across the output capacitor C11 is provided to a load (not shown). However, the rectifying diodes that constitute the bridge rectifier BR are inevitable to cause considerable conduction loss. As a result, the conversion efficiency of the converter is degraded. Moreover, anytime the boost inductor L11 is storing energy or releasing energy, three power semiconductor devices within the converter have to turn on to conduct currents, which would aggravate the conduction loss suffered by the converter.
FIG. 2 is a circuit diagram of a totem-pole power factor correction boost converter. In FIG. 2, a boost inductor L21 is connected between an input terminal and a junction node between the transistor switches Q21 and Q22. The converter of FIG. 2 further includes rectifying diodes D21 and D22 which are connected in series with each other and connected in parallel with the transistor switches Q21 and Q22, in which the switching of the transistor switches Q21 and the switching of the transistor switches Q22 are conducted in a complementary manner. The operation of the converter of FIG. 2 is described as follows. During the positive half-cycle of the input voltage, the transistor switch Q21 is turned on and the rectifying diode D21 is reverse-biased, and the output voltage of the converter is regulated by modulating the transistor switch Q22. During the time interval of D (of a duty cycle associated with the positive half-cycle), the transistor switch Q22 is turned on to connect the boost inductor L21 to the input AC voltage Vin. In the meantime, the inductor current of the boost inductor L21 rises up and thereby storing energy in the boost inductor L21 during the time interval of D. During the time interval of 1-D (of a duty cycle associated with the positive half-cycle), the transistor switch Q22 is turned off. The boost inductor L21 releases the stored energy to the output capacitor C21 through the rectifying diode D22, and thereby generating an output voltage across the output capacitor C21. During the negative half-cycle of the input voltage, the transistor switch Q22 is turned on and the rectifying diode D22 is reverse-biased, and the output voltage of the converter is regulated by modulating the transistor switch Q21. During the time interval of D (of a duty cycle associated with the negative half-cycle), the transistor switch Q21 is turned on to connect the boost inductor L21 to the input AC voltage Vin. In the meantime, the inductor current of the boost inductor L21 rises up and thereby storing energy in the boost inductor L21 during the time interval of D. During the time interval of 1-D (of a duty cycle associated with the negative half-cycle), the transistor switch Q21 is turned off. The boost inductor L21 releases the stored energy to the output capacitor C21 through the rectifying diode D21, and thereby generating an output voltage across the output capacitor C21. It can be understood from the above descriptions that only two power semiconductor devices are required to turn on to conduct currents during the positive half-cycle or the negative half-cycle. Therefore, the conversion efficiency of the converter of FIG. 2 is quite high. Moreover, when the converter of FIG. 2 is operating in the continuous conduction mode (CCM), the rectifying diode D21 or D22 is able to interconnect one end of the input AC voltage Vin and the output capacitor C21, so that the common-mode noise of the converter is quite low. Nonetheless, the parasite diodes on the transistor switches Q21 and Q22 of the converter shown in FIG. 2 are configured to switch at a high frequency, and their reverse recovery characteristics are quite bad. In practical applications, the converter is prone to lose the advantage of low common-mode noise due to the high switching frequency or the persistent operation in discontinuous conduction mode (DCM).
FIG. 3 is a circuit diagram of a conventional bridgeless power factor correction converter. In FIG. 3, boost inductors L31, L32 are connected in parallel and respectively connected to one end of the input AC voltage vin. Transistor switches Q31, Q32 are respectively connected to the boost inductors L31, L32. Rectifying diodes D31, D32 are respectively connected in series with the transistor switches Q31, Q32. The rectifying diodes D31, D32 are connected to the output capacitor C31 through a first bus and the transistor switches Q31, Q32 are connected to the output capacitor C31 through a second bus. During the positive half-cycle of the input AC voltage Vin, the transistor switch Q31 is turned on and an input current is induced to flow toward the boost inductor L31 so as to charge the boost inductor L31. In the meantime, the transistor switch Q32 is also turned on and the current path is closed through the body diode of the transistor switch Q32. Next, the transistor switch Q31 is turned off and the energy stored in the boost inductor L31 is discharged to the output capacitor C31 through the rectifying diode D31. The current path is closed through the body diode of the transistor switch Q32. During the negative half-cycle of the input AC voltage Vin, the transistor switch Q32 is turned on and an input current is induced to flow toward the boost inductor L32 so as to charge the boost inductor L32. In the meantime, the transistor switch Q31 is also turned on and the current path is closed through the body diode of the transistor switch Q31. Next, the transistor switch Q32 is turned off and the energy stored in the boost inductor L32 is discharged to the output capacitor C31 through the rectifying diode D32. The current path is closed through the body diode of the transistor switch Q31. Hence, during each half-cycle of the input AC voltage Vin, one transistor switch acts as an active switch and the other transistor switch acts as a rectifying diode. The major disadvantage of the converter shown in FIG. 3 is that power bounce would occur between the input voltage and the output voltage. More disadvantageously, the common-mode noise would be augmented with the increase of the parasitical capacitance value between the buses and ground.
FIG. 4 is modified circuit diagram of the bridgeless power factor correction converter of FIG. 3. Compared to FIG. 3, the configuration and operation of the boost inductors L41, L42, the transistor switches Q41, Q42, the rectifying diodes D41, D42, and the output capacitor C41 of FIG. 4 are similar to the configuration and operation of the counterparts of FIG. 3. Particularly, the bridgeless PFC converter of FIG. 4 adds a pair of auxiliary diodes D43, D44 to the input side of the converter for efficiently suppressing the common-mode noise of the converter. Nonetheless, the boost inductors L41, L41 would cause a waste of circuit board space due to its bulky dimensions. Also, the boost inductors L41, L42 are operating in an alternate manner during the time period of the input AC voltage Vin, which would result in a low utilization of the boost inductors L41, L42 and become difficult to improve the power density.
There is a tendency to develop a bridgeless power factor correction converter that can reduce the common-mode noise of the converter and enhance the power density of the converter.